Nowadays, polymer materials are frequently used for pipes for various purposes, such as fluid transport, i.e. transport of liquid or gas, e.g. water or natural gas, during which the fluid can be pressurised. Moreover, the transported fluid may have varying temperatures, usually within the temperature range from about 0° C. to about 50° C. Such pipes are preferably made of polyolefin plastic, usually unimodal polyethylene such as medium density polyethylene (MDPE; density: 0.930-0.942 g/cm3) and high density polyethylene (HDPE; density: 0.945-0.965 g/cm3).
According to WO 00/01765 a polymer composition for pipes is known comprising a multimodal polyethylene with a density of 0.930-0.965 g/cm3, an MFR5 of 0.2-1.2 g/10 min, an Mn of 8000-15000, an Mw of 180-330×103, and an Mw/Mn of 20-35, said multimodal polyethylene comprising a low molecular weight (LMW) ethylene homopolymer fraction and a high molecular weight (HMW) ethylene copolymer fraction, said HMW fraction having a lower molecular weight limit of 3500, and a weight ratio of the LMW fraction to the HMW fraction of (35-55):(65:45). The polymer composition according to WO 00/01765 is intended for pressurised pipes for the transport of gases and liquids such as cold water. This composition is not suitable for pipes for hot fluids such as hot water, according to present hot water polyethylene pipe standards, such as DIN 16 833.
By the expression “hot fluid” used herein is meant a gas or a liquid, usually water, having a temperature of at least 60° C., usually 60-100° C., such as 70-90° C.
Because of the high temperatures (at least 60° C.) involved, polymer pipes for hot fluids such as hot water, represent a particularly problematic type of polymer pipe. Not only must a polymer pipe for hot fluids such as hot water, fulfil the requirements necessary for other ordinary polymer pipes, such as cold water pipes, but in addition it must withstand the strain associated with hot fluid, usually hot water. The temperature of the hot water in a hot water pipe is normally at least 60-70° C. which means that the pipe must be able to withstand a higher temperature than that for a secure long term use. According to the standard DIN 16 833 a hot water pipe must meet the requirement of at least 165 h before failure at 95° C. and 3.6 MPa pressure. In this connection it should be understood that an increase of 10° C. of the water temperature means a decrease of the working life of the pipe by roughly 50%.
A specific type of hot water polymer pipe that involves even greater problems is floor heating pipes such as for the heating of bath room floors, because of the added requirement for high flexibility.
A polymer composition for pipes e.g. like that according to WO 00/01765 above, which is suitable for a cold water pipe is therefore not necessarily useful as a composition for pipes for hot fluids where the temperature of the fluid (usually water) could be 30-40° C. higher or more.
Up to now polyolefine pipes for hot fluids such as hot water have usually not been made of polyethylene polymers due to the fact that these materials are relatively prone to stress cracking at elevated temperatures when subjected to stress. Further, to sustain pressures usually used in hot water systems high density polyethylenes have to be used and these are not suitable for pipes with high flexibility such as hot water pipes for floor heating.
It would be desirable to achieve a pipe of polyethylene that fulfils the temperature and pressure as well as the flexibility requirements for a hot water pipe.